1. Field of the Invention
The present invention relates to provision of information regarding location of a mobile user equipment, and in particular to compensation of inaccuracies in timing information used in determining the location of a mobile user equipment such as a mobile station.
2. Description of the Related Art
In wireless communication systems such as those known as cellular communications systems a mobile user equipment such as a mobile station communicates with other stations via a wireless interface. In a cellular system the mobile user equipment such as a portable handset or other mobile station is served by radio access entities referred to as cells, hence the name cellular system. In a cellular system, the cell is provided by means of a base transceiver station (BTS). A base station may provide more than one cell and a cell may be provided by more that one base station. E.g. the 3rd generation telecommunications standards may refer to the base station as node B. Regardless the standard, the term base station will be used in this document to encompass any such element of an access entity which transmits to and/or receives signals from a mobile user equipment or the like via an air interface.
As the approximate size and the shape of the cell is known, it is possible to associate the cell to a geographical area. The size and shape of the cells may vary from cell to cell. One or several cells may also be grouped together to form a service area (SA).
Examples of cellular communication standards, without limiting this specification to these, include the second generation (2G) GSM (Global System for Mobile communications) or various GSM based systems (such as 2G GPRS: General Packet Radio Service), the AMPS (American Mobile Phone System), the DAMPS (Digital AMPS) or 3rd generation (3G) cellular system systems, such as the 3G GPRS and communication systems based on the WCDMA (Wideband Code Division Multiple Access), for example the UMTS (Universal Mobile Telecommunications System), IMT 2000, i-Phone and so on.
The cellular network apparatus and/or the mobile station can be employed for provision of location information of the mobile station and thus the user thereof. More particularly, the cells or similar geographically limited service areas and associated controller nodes facilitate the cellular system to produce at least a rough location information estimate concerning the current geographical location of a particular mobile station. If the location of the access entity is known, it is possible to conclude at least roughly from this information the geographical area in which the given mobile user equipment (communicating in said access entity) is likely to be at a given moment. A mobile user equipment may also be provided with appropriate equipment to generate information on which the positioning of the mobile user equipment can be based on.
Use of satellite based positioning systems for positioning of mobile stations has also been proposed. A well established satellite based positioning system is the GPS (global positioning system). Another example of the satellite based positioning system is the proposed Galileo™ location system. In the satellite based systems the positioning information is provided by means of computations that are based on the locations of the satellites and the timing of the positioning system. The timing arrangement is such that each satellite is synchronised to the one system time, or “universal time”. In the following the timing of the GPS system will be referred to as the ‘GPS time’.
In operation, each GPS satellite continuously transmits a bit stream containing the satellite identity, the GPS time and satellite trajectory models. The trajectory models may comprise e.g. an approximate long term model (almanac) and an accurate short term model (ephemeris). A GPS receiver can then estimate its location in three dimensional space based on the signals, typically from at least four different GPS satellites, the GPS time and other information such as the trajectory models.
A mobile station may be provided with a GPS receiver. The GPS receiver is adapted to search for the satellites, and he receive the GPS signals from the found satellites. Location determinations may then be performed at the mobile station based on the information signal received from the satellites.
A further development in the field of the GPS and the positioning of mobile stations is the so called Assisted Global Positioning System (A-GPS). The basic idea of the A-GPS is that the performance of a GPS receiver of a mobile station is enhanced by sending appropriate assistance data to the mobile station. Instead of sending such data from the satellites, the assistance data is transmitted via the air interface between the mobile station and the cellular network.
Various information may be provided as the assistance data. One of the components of the assistance data may be the GPS time. The GPS time can be used in assisting the GPS receiver to find more easily signals from GPS satellites. Further gain is obtained if the GPS time can be delivered to the mobile station (and its GPS receiver) as accurately as possible, as this will make the computation more accurate. Furthermore, less than four satellites may be enough for accurate positioning of the mobile station. The A-GPS is supported by various telecommunication standards, such as the 2G GSM and 3G UMTS specifications.
One possibility to transfer the GPS time to the mobile station is to send information about the correspondence of the GPS time relative to a timing used by the cellular system. That is, the mobile station is provided with information by means of which the GPS time can be tied to a certain event in the cellular system. This timing will herein be referred to as the cellular time.
The association of the GPS time and the cellular time is already supported, for example, by the GSM standards. For example, GSM specification No. 04.35 (version 8.1.0) describes broadcasting of GPS assistance data to user equipment. In chapter 4.2.1.6 a so called ‘Reference Time Information Element’ is defined as an information element (IE) that specifies the relationship between the GPS time and air-interface timing of the BTS transmission in the serving cell. In GSM specification No. 04.31 the corresponding point-to-point information is defined as fields that specify the relationship between the GPS time and air-interface timing of the BTS transmission in a reference cell.
Thus in the assisted GPS mobile location method a way of provision of the assistance data is to transfer the GPS time to the mobile by expressing the correspondence of the GPS time relative to a certain event in the cellular system. It has been proposed that in the GSM and UMTS standards the event comprises the moment of transmission of a certain signal. That is, the GPS time is expressed as a transmission moment of a certain cellular signal.
However, the Inventor has found that a problem may arise since the mobile user equipment can only detect the reception time. Since there will be at least some distance between the mobile user equipment and the transmitting base station, the moment of transmission will be different from the moment of reception due to the propagation delay. The mobile user equipment cannot relate the reception time to the transmission time of the relevant cellular signal, and thus the propagation delay may cause substantial inaccuracy in the delivered GPS time.
Best improvement in the location determination accuracy is believed to require transferring of the GPS time with an error that is less than 10 micro seconds. However, for example, in the GSM one bit period is 3.69 micro-seconds. This accuracy corresponds to 1.1 km in distance. The maximum cell size can be 35 km in a typical GSM system. Extended cell radius may also be used, these providing as large radiuses as 70 km. Thus the maximum possible propagation delay can be 116 microseconds. The delay is ten times more than the 10 micro second requirement, and equals about 34.6 km in distance.